It is expected that a device using an organic substance develops to a wide range of elementary elements such as an organic electroluminescent element (hereinafter referred to as an organic EL element), an organic transistor, an organic solar battery, and an organic semiconductor and uses. In addition to the above devices, devices having a positive hole injection transport layer include a quantum dot light emitting element, an oxide compound solar battery, etc.
An organic EL element is a charge injection type self light emitting device, utilizing the light emission generated at the time of recombining an electron and a positive hole reaching at a light emitting layer. Such an organic EL element has been developed actively since 1987 when T. W. Tang, et al. proved that an element comprising laminated thin films of a fluorescent metal chelate complex and a diamine based molecule emits light of high luminance with a low driving voltage.
The element configuration of the organic EL element comprises a cathode/an organic layer/an anode. The organic layer in an initial organic EL element has a two layer structure comprising a light emitting layer and a positive hole injection layer. At present, however, in order to obtain a high light emitting efficiency and a long driving lifetime, various multilayered structures such as a five layer structure comprising an electron injection layer/an electron transport layer/a light emitting layer/a positive hole transport layer/a positive hole injection layer, etc. have been proposed.
It is said that the layers other than the light emitting layer including the electron injection layer, the electron transport layer, the positive hole transport layer and the positive hole injection layer have effects that charges are easily injected and transported to the light emitting layer, the balance between an electronic current and a positive hole current is maintained by blocking charges, and the diffusion of a light energy exciton is prevented.
For the purpose of improving charge transport ability and charge injection ability, there has been attempts to increase electric conductivity by mixing an oxidizing compound to a positive hole transport material (Patent Literatures 1 and 2).
In Patent Literature 1, as the oxidizing compound, that is, an electron accepting compound, a compound containing counter anions such as a triphenylamine derivative and antimony hexafluoride, and a compound having a significantly-high electron-accepting property, in which a cyano group is bonded to carbon of a carbon-carbon double bond, such as 7,7,8,8-tetracyanoquinodimethane, are used.
In Patent Literature 2, as an oxidizing dopant, a general oxidant can be exemplified, and also halogenated metal, Lewis acid, organic acid and salt of arylamine and halogenated metal or Lewis acid can be exemplified.
In Patent Literatures 3 to 6, as the oxidizing compound, that is, the electron accepting compound, a metal oxide being a compound semiconductor is used. For the purpose of obtaining a positive hole injection layer having an excellent injection property and charge transfer property, for example, a thin film is formed by a vapor deposition method using a metal oxide such as vanadium pentoxide or molybdenum trioxide, or a mixed film is formed by codeposition of a molybdenum oxide and an amine based low molecular weight compound.
In Patent Literature 7, a solution, in which oxovanadium(V)tri-1-propoxideoxide is dissolved as the oxidizing compound, that is, the electron accepting compound, is used. Patent Literature 7 discloses a method for forming a charge transfer complex comprising the step of forming a mixed coating film of the solution and a positive hole transport polymer followed by hydrolysis in water vapor to obtain vanadium oxide.
Patent Literature 8 discloses that an organic EL element having a long lifetime is produced by dispersing particles produced by physically pulverizing molybdenum trioxide in a solution to produce a slurry, and applying the slurry to form a positive hole injection layer, as an attempt to form a coating film of molybdenum trioxide.
On the other hand, an organic transistor is a thin film transistor using an organic semiconductor material comprising an organic polymer or an organic low-molecular compound having a π conjugated system for a channel area. A general organic transistor comprises a substrate, a gate electrode, a gate insulating layer, source and drain electrodes and an organic semiconductor layer. In the organic transistor, by changing a voltage (gate voltage) applied to the gate electrode, a quantity of electric charge of an interface of a gate insulating layer and an organic semiconductor layer is controlled and switching is performed by changing a current value between a source electrode and a drain electrode.
As an attempt to improve an on-current value of the organic transistor and stabilize element characteristic by lowering a charge injection barrier between the organic semiconductor layer and the source electrode or the drain electrode, it is known that the carrier density in the organic semiconductor layer in the vicinity of the electrode is increased by introducing the charge transfer complex to an organic semiconductor (for example, Patent Literature 9).